During combustion of fuels in a fluidized bed, the power output is usually controlled by changing the bed depth. With conventional fluidized bed boilers for the production and superheating of steam, which is expanded in a steam turbine, the control of the bed depth in case of changes of the power output entails the transportation of large quantities of bed material back and forth to the fluidized bed. These large material flows require a complicated transport systems involving, among others, intermediate storage containers for bed material. In addition, the considerable bed depth adjustments and the associated material flows as well as the exposure of large heat transfer surfaces result in a slow control and in the temperature of the flue gases falling in case of partial load, which is reflected in inferior environmental performance.
If the fluidized bed is pressurized, that is, is included in a plant for combustion in a pressurized fluidized bed, a PFBC (Pressurized Fluidized Bed Combustion) plant, in which energy is recovered also from the hot pressurized flue gases by means of gas turbines, conventional control of the power output with bed depth adjustments also results in the efficiency of the plant decreasing with decreasing flue gas temperature as well as in considerable difficulties in adjusting the power balance between the steam and gas turbines during operation.